In order to relieve impact on the human body in a car accident, mounting of an airbag in a vehicle is proceeding. As an airbag capable of inflating by gas or the like and absorbing impact on the human body in a collision, for the protection of an occupant, a curtain airbag, a side airbag, a knee airbag, a rear airbag and the like are being put into practice, in addition to a driver's sheet airbag and a passenger sheet airbag. Furthermore, for the protection of a pedestrian, mounting of various airbags such as an airbag on the outside of the vehicle is in discussion. On the other hand, with the growing interest in environmental problems, from the standpoint of improving fuel efficiency of a vehicle, weight saving is also required of an airbag module. Also, in order to enhance fuel efficiency or energy efficiency by reducing vehicle size, the region in which the airbag module is stored is becoming narrower, and more reduced size is necessary.
The airbag module mainly comprises an airbag obtained by forming a fabric composed of a synthetic fiber into a bag shape, an inflator for generating gas capable of deploying the airbag, and a device for detecting a collision and controlling the deployment. Of these members, the inflator is composed of a strong container for housing a propellant in the container and generating a high-pressure high-speed gas therefrom, or composed of a strong high-pressure gas container for housing a high-pressure gas in the container and generating a high-pressure high-speed gas by opening the container with an explosive. Accordingly, its weight or volume accounts for a large part of the airbag module.
As for the airbag, in order to reduce the weight thereof, reducing the fineness of the fiber constituting the airbag fabric or fabrication of a non-coated airbag by using an airbag fabric substantially uncoated with a resin or an elastomer is being carried out.
Compared with a coated airbag, the deployment gas in the non-coated airbag is not sufficiently utilized. In an airbag for front collision accident, such as a driver's seat airbag and passenger seat airbag having a long history of airbag mounting, after the airbag is deployed by gas, the gas is released from a vent hole provided in the airbag or a filter fabric part so as to receive the human body and thereby the impact energy is absorbed. Accordingly, the amount of leakage of the deployment gas in the non-coated airbag is not strictly taken as a problem. However, in recent years, a change in the deployment size of the airbag by controlling the opening of the vent hole or employing an airbag undergoing stepwise the deployment is proceeding. Further, mounting a side curtain airbag aimed at quick deployment without the vent hole is being employed. In turn, also as the non-coated airbag, an airbag ensuring more reduction in the loss of the deployment gas during deployment than ever is demanded. Furthermore, when there is no loss of the deployment gas, the volume of the inflator need not be excessively increased and the airbag module can be expected to be reduced in the size.
With respect to the air permeability of the non-coated airbag under high pressure, Patent Document 1 discloses a technique demonstrating that higher air permeability is more effective in reducing the impart, but the desire is to increase the gas utilization efficiency by suppressing the air permeability under high pressure as much as possible. Also, Patent Document 2 discloses an airbag technique of fabricating a non-coated airbag by using a fabric composed of a polyester filament yarn to cause no change in the permeability or bursting strength under a high-temperature high-humidity environment, but in a polyester fabric, a melted hole may be produced due to burning residue from the propellant in the inflator and a rupture may be caused, and it is demanded to fabricate an airbag fabric ensuring lower air permeability under high pressure by using a polyamide fabric excellent in heat resistance.
With respect to the pressure resistance of the airbag after a heat treatment, Patent Document 3 describes excellent tear tenacity retention of a polycapramide fiber after a heat treatment, but for preventing the airbag from melting and rupturing due to a high-temperature gas or reaction residue in case of using an explosive for the inflation gas, the fiber above is inferior to the polyamide 6·6 fiber in view of melting temperature. In the fabric composed of a polycapramide fiber, bag deployment by an explosive is improper. Patent Document 4 describes a technique of enhancing the tear tenacity by imparting a polysiloxane-based softener by immersion, but it is unprofitable to specially use a treating agent or increase the processing step. Also, the weaving yarn in the sewn part is slid off to cause fiber dropout, and this impairs the strength of the sewn part. Patent Document 5 demonstrates that in the finish oil-imparted fabric without scouring, the tear tenacity may be maintained, but this document is silent about the characteristics such that the deployment speed is maintained after a heat treatment. Patent Document 6 demonstrates that by incorporating a thermal stabilizer into a polycapramide fiber to make an original yarn allowing generation of a thermal shrinkage stress at a higher temperature, the increase in air permeability of the airbag base cloth after a heat treatment can be suppressed. However, even if low air permeability is maintained after a heat treatment, characteristics necessary to maintain the deployment speed are not described.
Conventionally, for the airbag mounted in the interior of an automobile, i.e., in a cabin, it is important that the airbag stored for a long term under the conditions of summer and daytime high temperatures or winter and nighttime low temperatures maintains the deployment performance such as pressure resistance at the actuation of inflation and deployment after the elapse of days. However, the place in which an airbag loaded to inflate toward the outside of the vehicle for the protection of a pedestrian is mainly mounted is outside the cabin and particularly, when the place is near the engine room inside the bonnet, the airbag is exposed to a harsher environmental condition. That is, durability of the airbag under a harsh environment is more stringently required. The task is to maintain the deployment performance after the elapse of days under harsher environmental conditions.
The airbag for the protection of a pedestrian takes such a large area and a large dimension as covering the front part of the bonnet or the lower part of the front glass. Also, compared with the case of relieving the short distance collision in an automobile, large inflation is used. Accordingly, such an airbag is a large-volume airbag, but the deployment speed of the airbag stored for a long term is disadvantageously slowed after many days. Even when a proper deployment timing of the airbag is adjusted by a detector or a deployment/ignition control device, if the timing is lagged because of, for example, a delay caused until the gas functions by permeating from end to end during deployment of the bag, the impact absorbing performance is deteriorated. Accordingly, the task is also to maintain the deployment speed of a large-volume airbag under harsh environmental condition.